Cattell's methodological rigor relied on what he termed the T-Data (TETRAD) approach, integrating three distinct data collection modalities to ensure comprehensive personality assessment without method-variance contamination:
- L-Data (Life Record Data): Objective records of behavior in everyday natural settings, such as school grades, traffic violations, or employment ratings.
- Q-Data (Questionnaire Data): Self-evaluation data obtained through introspection, rating scales, and inventories where individuals report on their own feelings, habits, and attitudes.
- T-Data (Test Data): Objective, standardized performance tests where the individual is unaware of what aspect of behavior is being evaluated (e.g., projective techniques, physiological stress tests, cognitive speed tasks), minimizing social desirability bias.
Cattell also applied factor analysis to cognitive abilities, distinguishing between Fluid Intelligence (Gf)—the neurophysiological capacity for logical problem-solving independent of acquired education—and Crystallized Intelligence (Gc)—accumulated knowledge, vocabulary, and culturally transmitted skills.